1. Field of the Invention
This invention relates to optical disc drives, and more particularly, to a prism-type objective lens for use in the pickup head of an optical disc drive capable of driving at least two types of optical discs such as a CD (compact disc) and a DVD (digital video disc). The prism-type objective lens is structured in such a manner that allows the pickup head to be made more compact in size with reduced structural complexity.
2. Description of Related Art
In the multimedia age, an optical disc drive is an indispensable device on a personal computer system. Conventional optical disc drives are designed to read data from a low-density optical disc, often referred to as a CD (compact disc). However, with advances in optical technology, a new type of high-density optical disc, called a DVD (digital video disc) is able to hold much more data than the conventional CD can hold. New types of optical disc drives are usually designed to be capable of reading data from both types of optical discs. With notebook computers getting more popular, a DVD/CD optical disc drive should be made compact enough so that it can be used as a standard built-in device in a notebook computer. Presently, there is one type of pickup head which is only 6.7 mm (millimeter) in height; however, it is still considered too thick to be used as a DVD/CD drive for installation on a notebook computer. Conventional optical disc drives are quite complex in structure due to the fact that each single optical component can only provide one single optical function; for example, a folding mirror is used solely for reflecting the light incident on it to other directions, and a collimator is solely used to collimate the light passing through it. As a result, the total number of the optical components constituting the pickup head is large, thus increasing the structural complexity of the optical system of the pickup head. Several conventional optical disc drives that are too bulky in size due to the use of too many optical components are illustratively depicted in the following.
FIGS. 1A-1B are schematic top and side views of a conventional DVD pickup head that is manufactured by the SANYO Corporation. As shown, this DVD pickup head includes, which is of the type using a liquid crystal (LC) shutter to set the desired numerical aperture (NA). As shown, this DVD pickup head includes a laser diode 15, a grating 14, a photo-detector pre-amp IC 10, a board splitter 11, a collimator 12, an objective lens 13, a folding mirror 16, and a polarizer 17 (FIG. 1B). In operation, the laser diode 15 generates a laser beam, which then passes through the grating 14 to the board splitter 11. At the board splitter 11, part of the laser beam is reflected to the collimator 12 and collimated by the same. The collimated light from the collimator 12 is then reflected by the folding mirror 16 to the polarizer 17. The output light from the polarizer 17 is then focused by the objective lens 13 onto the information surface of the optical disc (not shown) being read. The reflected light from the optical disc (not shown) then propagates in the reverse direction through the polarizer 17, the folding mirror 16, and the collimator 12 to the board splitter 11. At the board splitter 11, part of the reflected light transmits straight through the board splitter 11 to the photo-detector pre-amp IC 10. In response to the reflected light, the photo-detector pre-amp IC 10 generates a number of opto-electrical signals that can be processed in a conventional method to obtain the data stored on the optical disc.
FIGS. 2A-2B are schematic top and side views of a conventional DVD pickup head that is manufactured by the PIONEER Corporation. As shown, this DVD pickup head includes a laser diode 20, a grating 21, a first folding mirror 22, a board splitter 23, a quarter-wave plate 24, a collimator 25, a second folding mirror 26, a holographic optical element (HOE) 27, an objective lens 28, and a photo-detector pre-amp IC 29. The laser diode 20 is capable of generating a laser beam of 650 nm (nanometer) in wavelength. The laser beam from the laser diode 20 first passes through the grating 21 to the first folding mirror 22 where the laser beam is reflected to the board splitter 23. At the board splitter 23, part of the laser beam propagates straight through the board splitter 23 to the quarter-wave plate 24. The output light from the quarter-wave plate 24 is then collimated by the collimator 25 and then reflected by the second folding mirror 26 to the HOE 27. The output light from the HOE 27 is then focused by the objective lens 28 onto the information surface of the optical disc (not shown) being read. This DVD pickup head can be selectively set to read either a CD or a DVD. The reflected light from the optical disc (not shown) then propagates in the reverse direction through the HOE 27, the second folding mirror 26, the collimator 25, and the quarter-wave plate 24 to the board splitter 23. At the board splitter 23, part of the reflected light from the optical disc is reflected sideways to the photo-detector pre-amp IC 29. In response to the reflected light, the photo-detector pre-amp IC 29 generates a number of opto-electrical signals that can be processed in a conventional method to obtain the data stored on the optical disc.
FIGS. 3A-3B are schematic top and side views of a conventional DVD pickup head that is manufactured by the PANASONIC Corporation. As shown, this DVD pickup head includes a laser diode 30, a grating 31, a collimator 32, a board splitter 33, a first folding mirror 34, an objective lens 35 having a built-in HOE, a second folding mirror 36, and a photo-detector pre-amp IC 37. In operation, the laser diode 30 generates a laser beam which then passes through the grating 31 and the collimator 32 to the board splitter 33. At the board splitter 33, part of the laser beam propagates straight through the board splitter 33 to the first folding mirror 34 where the laser beam is reflected to the objective lens 35. The built-in HOE of the objective lens 35 causes a holographic effect on the laser beam, and then the objective lens 35 focuses the laser beam onto the information surface of the optical disc (not shown) being read. This DVD pickup head can be selectively set to read either a CD or a DVD. The reflected light from the optical disc (not shown) then propagates in the reverse direction through the objective lens 35 and subsequently through the first folding mirror 34 to the board splitter 33. At the board splitter 33, part of the reflected light from the optical disc is reflected sideways to the second folding mirror 36 which then reflects the beam to the photo-detector pre-amp IC 37. In response to the reflected light, the photo-detector pre-amp IC 37 generates a number of opto-electrical signals that can be processed in a conventional method to obtain the data stored on the optical disc.
FIGS. 4A-4B are schematic top and side views of a conventional DVD pickup head that is manufactured by the SANYO Corporation. As shown, this DVD pickup head includes a laser diode 40, a grating 41, a board splitter 42, a folding mirror 43, a collimator 44, a lens unit which can be switched between a DVD objective lens 45 and a CD objective lens 46 (in the case of reading a DVD, the DVD objective lens 45 is set in position, while in the case of reading a CD, the CD objective lens 46 is set in position), and a photo-detector pre-amp IC 47. In operation, the laser diode 40 generates a laser beam, which then passes through the grating 41 to the board splitter 42. At the board splitter 42, part of the laser beam transmits straight through the board splitter 42 to the folding mirror 43 where the laser beam is reflected to the collimator 44. Depending on the current type of the optical disc being inserted in the optical disc drive, either the DVD objective lens 45 or the CD objective lens 46 is set in position in the optical axis of the collimator 44, allowing the output light from the collimator 44 to be focused by the corresponding type of objective lens onto the information surface of the optical disc (not shown). The reflected light from the optical disc (not shown) then propagates in the reverse direction through the objective lens 45 or 46, the collimator 44 and the folding mirror 43 to the board splitter 42. At the board splitter 42, part of the reflected light is reflected sideways to the photo-detector pre-amp IC 47. In response to the reflected light, the photo-detector pre-amp IC 47 generates a number of opto-electrical signals that can be processed in a conventional method to obtain the data stored on the optical disc.
FIGS. 5A-5B are schematic top and side views of a conventional DVD pickup head that is manufactured by the SAMSUNG Corporation. As shown, this DVD pickup head includes a laser diode 50, a grating 51, a board splitter 52, a folding mirror 53, a collimator 54, an objective lens 55, and a photo-detector pre-amp IC 56. The objective lens 55, in particular, is of an annular mask type. In operation, the laser diode 50 generates a laser beam, which then passes through the grating 51 to the board splitter 52. At the board splitter 52, part of the laser beam transmits straight through the board splitter 52 to the folding mirror 53 where the laser beam is reflected to the collimator 54. The collimated light from the collimator 54 is then focused by the objective lens 55 onto the information surface of the optical disc (not shown) being read. This DVD pickup head can be selectively set to read either a CD or a DVD. The reflected light from the optical disc (not shown) then propagates in the reverse direction through the objective lens 55, the collimator 54 and the folding mirror 53 to the board splitter 52. At the board splitter 52, part of the reflected light is reflected sideways to the photo-detector pre-amp IC 56. In response to the reflected light, the photo-detector pre-amp IC 56 generates a number of opto-electrical signals that can be processed in a conventional method to obtain the data stored on the optical disc.
In the foregoing five conventional models of DVD pickup heads, a common design scheme is the use of an objective lens in conjunction with a folding mirror, with the folding mirror being used to reflect the laser beam to the objective lens. One drawback to this design scheme, however, is that it makes the overall pickup head bulky in size since the objective lens and the folding mirror are separate components. Moreover, the use of the board splitters, the polarizers, and the gratings also make the foregoing pickup heads bulky in size and complex in structure. These drawbacks make these conventional pickup heads less compact and therefore unsuitable for use on an optical disc drive that is intended to be installed on a notebook computer.